1. Field of the Invention
The present invention relates to an image compression apparatus and an image expansion apparatus, which compresses/expands image data, particularly relates to an image compression apparatus and an image expansion apparatus, which compresses/expands image to be compressed by a block unit having a predetermined number of pixels.
2. Description of Related Art
In an image on a sheet, a character area and a photographic area are intermingled in many cases, and when the image on the sheet is evenly compressed/expanded with a compression technology, image quality may greatly deteriorate in a specific area. Then, the image processing apparatus, which discriminates the characteristic of the image per block and switches the compression technology for each block, is proposed (for example, refer to Japanese Patent Application Publication No. H11-75069). In this apparatus, ADCT (Adaptive Discrete Cosine Transform) or BTC compression is selected corresponding to the characteristic of the image for each block as a compression technology.
In order to prevent the degradation of the image quality, it is preferred to switch the compression technology corresponding to the characteristic of the image of each block. However, when information for discriminating the compression technology per block is separately added, the data volume of that portion will increase and reduction of the compression ratio will occur. In the image processing apparatus described in Japanese Patent Application Publication No. H11-75069, when the compression technology is switched, data volume after compression respectively differs and there is a problem that the management of the breaks of the block becomes complicated.
An object of the present invention is to provide an image compression apparatus and an image compression expansion apparatus, which can switch a compression technology corresponding to the characteristic of an image of each block without causing an increase of data volume for discriminating the compression technology to solve the above-mentioned problem.
Further, another object is to provide an image compression apparatus and an image compression expansion apparatus, in which the data volume of compressed data becomes uniform even in a case where the compression technology is switched.